Composites incorporating nanoparticles (NPs) within metal-organic frameworks (MOFs) find applications in many different fields. In particular, using MOF layers as molecular sieves built on the NPs could enable selectivity in heterogeneous catalysis. However, such composites typically exhibit low catalytic efficiency, due to the slow diffusion of the reactants in the long and narrow channels of the MOF shell. In order to improve the catalytic efficiency of these systems, here we report the fabrication of NPs incorporated in nanosized MOFs (NPs@nano-MOFs), obtained by reducing the size of the MOF crystals grown around the NPs. The crystal size of the composites was controlled by modulating the nucleation rate of the MOFs during the encapsulation of pre-synthesized and catalytically active NPs; in this way, NPs@MOF crystals smaller than 50 nm were synthesized and subsequently used as highly efficient catalysts. Due to the shorter path from the MOF surface to the active sites, the obtained Pt@nano-MOFs composites showed a higher conversion rate than their larger-sized counterparts in the synthesis of imines via cascade reaction of nitrobenzene and in the hydrogenation of olefins, while retaining the excellent size and shape selectivity associated with the molecular sieving effect of the MOF layer. The present strategy can also be applied to prepare other encapsulated nanostructures combining various types of NPs and nano-MOFs, thus highlighting the broad potential of this approach for developing optimized catalysts with high reactivity and selectivity.

Zinc-air batteries have recently attracted considerable interest owing to the larger storage capacity and lower cost compared to their lithium-ion counterparts. Electrode catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play a critical role in the operation of rechargeable zinc-air batteries. Herein, we report a simple and scalable strategy to fabricate porous carbon polyhedra using Zn-doped Co-based zeolitic imidazolate frameworks (ZnCo-ZIFs) as precursors. Strikingly, Zn doping leads to smaller Co nanoparticles and higher nitrogen content, which in turn enhances the ORR and OER activities of the obtained porous carbon polyhedra. The synergistic effect of the N-doped carbon and cobalt nanoparticles in the composite, the improved conductivity resulting from the high graphitization of carbon, and the large surface area of the porous polyhedral structure resulted in porous carbon polyhedra with excellent ORR and OER electrocatalytic activity in alkaline media. More importantly, air cathodes based on the optimal porous carbon polyhedra further exhibited superior performance to Pt/C catalysts in primary and rechargeable zinc-air batteries.

High quality gold nanorods (NRs) with a monodisperse size and aspect ratio are essential for many applications. Here, we describe how nearly monodisperse gold NRs can be separated from polydisperse samples using density gradient ultracentrifugation. Size and dimension analysis by transmission electron microscopy (TEM) and absorption spectroscopy revealed that the Au NRs were separated mainly as a function of their aspect ratio. The surface-enhanced Raman scattering (SERS) activity of Au NRs with lower aspect ratio is notably stronger than that of NRs with higher aspect ratio under 633 nm laser excitation, due to the size-dependent absorption of the longitudinal plasmon band. The separation approach provides a method to improve the quality of NRs produced by large scale synthetic methods.